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US6980362B2 - Optical filters - Google Patents

Optical filters Download PDF

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Publication number
US6980362B2
US6980362B2 US10/433,217 US43321703A US6980362B2 US 6980362 B2 US6980362 B2 US 6980362B2 US 43321703 A US43321703 A US 43321703A US 6980362 B2 US6980362 B2 US 6980362B2
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US
United States
Prior art keywords
substrate
filter according
refractive index
resonator
filter
Prior art date
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Expired - Fee Related
Application number
US10/433,217
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English (en)
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US20040057144A1 (en
Inventor
Keith Loder Lewis
Gilbert William Smith
Mark Edward McNie
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Qinetiq Ltd
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Qinetiq Ltd
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Assigned to QINETIQ LIMITED reassignment QINETIQ LIMITED ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: LEWIS, KEITH LODER, MCNIE, MARK EDWARD, SMITH, GILBERT WILLIAM
Publication of US20040057144A1 publication Critical patent/US20040057144A1/en
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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/02—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the intensity of light
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/001—Optical devices or arrangements for the control of light using movable or deformable optical elements based on interference in an adjustable optical cavity
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/28—Interference filters
    • G02B5/284—Interference filters of etalon type comprising a resonant cavity other than a thin solid film, e.g. gas, air, solid plates
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29395—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29398—Temperature insensitivity
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction
    • G02F1/025—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/213—Fabry-Perot type

Definitions

  • WDM wavelength division multiplexing
  • components required for separating the different optical channels in the system e.g. for add/drop multiplexers are formed from fibre Bragg gratings or complex interference filter structures deposited by vacuum deposition.
  • the latter devices are usually based on multilayer stacks of two materials with contrasting refractive indices (e.g. titania/silica or tantala/silica), and the stack design is optimized around variants based on coupled Fabry Perot etalons, to realize the required passband characteristics, with bandwidths typically of the order of 0.4 nm at design wavelengths close to 1550 nm.
  • the filters are complex and require the deposition of several hundred discrete layers of material with tight control of film thickness.
  • Criteria for enhanced stability also drive the need to use fabrication techniques which ensure the deposition of films free from porosity and which are fully densified. Such devices are then cut to the required size (typically 1–2 mm square) and incorporated into the WDM device package. Separate filters are required for each of the discrete communication wavelengths across the 1500–1580 nm band. Despite such complexities, such devices have become standard for use in current networks.
  • these devices are manufactured using multi-step lithography/etching processes, to form the active device in a geometry conformal to the plane of silicon substrates.
  • the number of filter elements possible in such devices is small, which limits the degree of optical contrast achievable around the design wavelengths.
  • stress effects induced as a result of the fabrication of the silicon membrane structures result in curvature of the elements and consequent broadening of the spectral characteristics of the device.
  • Such filters are tuned by application of an electrostatic field or by exploiting thermo-optic effects arising from the temperature variation of the refractive index of silicon.
  • the invention involves a novel approach to the design and fabrication of micro-optical filters, including MOEMS-based filters, which overcomes most of the difficulties encountered in the conformal approach.
  • the invention provides an optical filter comprising a substrate in which is formed a plurality of parallel slots so as to define layers (slots and substrate laminae—the latter are also termed lamellae, but the former term will be used herein) of alternating refractive index.
  • the slots are generally perpendicular to a surface of the substrate, and in the embodiments they extend inwardly from a single surface of the substrate.
  • the said single surface may be formed with a groove on at least one side of the plurality of slots for accommodating the optic fibre(s).
  • a filter according to the invention may comprise a series of contiguous said layers of the form (aH bL) ⁇ x where aH denotes a layer of high refractive index provided by the substrate material, bL denotes a layer of low refractive index provided by a said slot, a and b denote respective integral numbers of quarter wave thicknesses of said layers, and x is an integer denoting the number of repetitions of the unit (aH bL).
  • the substrate may be of silicon, or a high refractive index semiconductor material. Other materials could be used provided they have a sufficient refractive index to be used in a practical device.
  • the invention embraces filter constructions which are not Fabry-Perot filters, and these can provide fairly broad wavelength characteristics useful in some device architectures.
  • preferred embodiments of the invention comprise a central Fabry-Perot cavity (or resonator region—the terms will be used interchangeably hereafter) with an optical thickness of an integral number of half wavelengths. This may be defined by a central slot, or by a central laminar portion of the substrate material between two slots. Further slots and substrate laminae either side of the cavity define interference structures for modifying the optical characteristics of the cavity as explained in more detail below in relation to the embodiments.
  • Means may be provided for altering the optical thickness of the central cavity to tune the filter.
  • this may comprise means for altering the physical thickness of the cavity, such as by an electrostatic comb drive, a piezoelectric drive, or the use of an actuator using a shape memory alloy.
  • its refractive index may be varied to tune the filter, by heating or cooling.
  • an electric current may be passed through an electrically conductive substrate for heating.
  • the substrate may be provided with a heating means such as thermoelectric means (also for cooling), or an electrically resistive means e.g. a film of metal alloy or indium tin oxide, through which electric current may be passed.
  • Tuning may be for setting up purposes, or for frequency selection in use.
  • the filter may be of the general form (aH bL) ⁇ x cH dL eH (fL gH) ⁇ y
  • aH, cH, eH and gH denote integral multiple quarter wave thick layers of high refractive index material provided by the substrate
  • bL, dL and fL denote integral multiple quarter wave thick layers of low refractive index provided by the slots.
  • the integers a, b, f and g denote an odd number of quarter waves, and in particular cases the values of a and b may equate to d and f respectively, while in other cases they may be different
  • c and e denote an integral odd number of quarter waves whereas d is an integral even number of quarter waves at the design wavelength of the cavity.
  • d is zero and cH and eH are taken together to provide an integral multiple half wavelength thickness (c+e)H.
  • the operators ⁇ x and ⁇ y indicate that the associated components (aH bL) and (fL gH) may be repeated a plurality of times x and y if appropriate.
  • Fabry-Perot filters constructed according to the invention may include at least one further Fabry-Perot cavity optically coupled to the said cavity, for example using the convention above a filter of the form (aH bL) ⁇ x aH dL aH (bL aH) ⁇ 2x dL aH (bL aH) ⁇ x, where a and b are different odd integers, and d is an even integer.
  • one cavity is tuned, for example, and thereafter the other cavity or cavities are tuned to match, again for example using one of the methods mentioned above. Transmission and filtering will only be expected to occur once matching has been achieved.
  • FIG. 1 shows in schematic perspective view a first general example of Fabry-Perot filter according to the invention, having a central air gap cavity in a silicon substrate;
  • FIG. 2 illustrates the optical transmission characteristics of a specific example of a filter of the type generally shown in FIG. 1 ;
  • FIG. 3 shows an experimental plot of reflectance against normalised wavelength
  • FIG. 4 is derived from a cross-sectional scanning electron micrograph of the filter to which FIG. 2 relates;
  • FIG. 5 illustrates the optical transmission characteristics of a second example of Fabry-Perot filter according to the invention which is based on a silicon—air gap structure and has a solid silicon central cavity spacer, and
  • FIG. 6 illustrates the optical transmission characteristics of a two cavity silicon-air gap Fabry-Perot etalon constructed according to the invention.
  • FIG. 1 shows a schematic diagram of a first embodiment of filter according to the invention.
  • a silicon substrate 1 is provided with a filter element 2 between a pair of V-bottomed grooves 6 for accommodating input and output optic fibres 7 .
  • the element 2 includes a central air cavity 3 between a mirror group 4 and a mirror group 5 .
  • the cavity 3 is essentially a slot extending into the substrate orthogonally from its surface, and each mirror group is defined by further slots which are parallel to the slot of cavity 3 and also extend orthogonally from the same substrate surface, whereby to leave thin substrate laminae spaced by air gaps.
  • Cavity 3 and the groups 4 and 5 are formed by exploiting deep-dry etching techniques such as those provided by the Bosch STS process, which enables the provision of etched vias with aspect ratios in excess of 30:1 under optimised conditions.
  • means are provided to ensure adequate collimation of light entering or leaving the fibres, for example precision lens elements fitted at the ends of the fibres.
  • This construction possesses an integral orthogonal geometry relative to the plane of the silicon substrate surface, and this presents many advantages in the realisation of effective devices, for example:
  • a conventional Fabry-Perot etalon would have a design of the form (HL) ⁇ 2 H 2L H (LH) ⁇ 2, where H and L denote integral quarter wave thicknesses of silicon and air respectively, and the operators ⁇ 2 indicate the number (2) of repetitions of the associated (HL) or (LH) component This would mean that the thicknesses of the individual components of the mirror would require to be about 110 nm for the silicon and 380 nm for the air gaps. Such dimensions are difficult to achieve on an accurate and reproducible basis using modem lithographic techniques and would severely limit the depth of etching.
  • FIG. 1 has the form (27H 9L) ⁇ 2 27H 8L 27H (9L 27H) ⁇ 2.
  • the spectral characteristics thereof are shown as a theoretical plot in FIG. 2 , where the central sharp curve 8 indicates transmittance and the broader curve 9 indicates optical density, and an experimental plot of reflectance against normalised wavelength is shown in FIG. 3 .
  • the silicon and air mirror elements are now close to 3 microns (27H) and 3.5 microns (8L) thick respectively.
  • FIG. 4 is a schematic showing derived from a scanning electron micrograph of a part of a cleaved device fabricated to this design highlighting the lack of curvature in the silicon mirror webs.
  • FIG. 5 shows the spectral characteristics of a design based on the sequence (27H 9L) ⁇ 3 28H (9L 27H) ⁇ 3.
  • FIG. 6 shows the spectral characteristics of such a device based on the sequence (37H 9L) 37H 8L (37H 9L) ⁇ 3 37H 8L 37H (9L 37H).
  • the out-of-band rejection characteristics are similar to those achieved in FIGS. 2 and 3 , but the pass-band has been broadened to allow for a degree of tolerancing in the positioning of the laser line.
  • Frequency agility can be achieved in a number of ways.
  • one of the mirror groups shown as group 5
  • one of the mirror groups could simply be moved by freeing the mirror assembly from its pedestal and providing electrostatic comb-drives (not shown) to position the mirror group at any desired position along the direction of the arrow A.
  • This also enables a degree of freedom in the trimming of the filter since it can now be moved in a continuous manner into correspondence with any required laser line as desired.
  • Other actuation techniques are possible, including the use of piezoelectric materials such as PZT, or shape memory alloys.
  • a digitally switched filter which is step-tunable to any of the communication laser lines, without passing through any other line.
  • Such a response characteristic can be achieved using the two-cavity device by step-wise tuning one cavity at a time. Transmission is blocked until both cavities coincide.
  • a considerable degree of frequency agility can also be effected by exploiting the variation in refractive index of materials with changes in temperature. This is most effective for devices based on solid silicon cavities and can be achieved simply by electrical current flow through the element. This obviates the need to shift the position of any of the mirror groups, so simplifying the construction of the device. Since the mass of the silicon spacer is relatively small, temporal response within 0.1 msec should be achievable provided that the degree of thermal isolation of the element is sufficient.
  • gaps in the substrate refers to air gaps
  • these gaps could be filled with a different material, for example a different fluid or a vacuum. Alteration of the material within the gaps could be used to alter the characteristics of the filter. However, it is preferred to keep the refractive index difference between the substrate and gaps relatively high for reasons outlined above.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Optical Filters (AREA)
  • Mechanical Light Control Or Optical Switches (AREA)
  • Optical Integrated Circuits (AREA)
US10/433,217 2000-11-30 2001-11-23 Optical filters Expired - Fee Related US6980362B2 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GBGB0029224.3A GB0029224D0 (en) 2000-11-30 2000-11-30 Optical filters
GB0029224.3 2000-11-30
PCT/GB2001/005152 WO2002044788A1 (en) 2000-11-30 2001-11-23 Optical filters

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US20040057144A1 US20040057144A1 (en) 2004-03-25
US6980362B2 true US6980362B2 (en) 2005-12-27

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US (1) US6980362B2 (ja)
EP (1) EP1337888B1 (ja)
JP (2) JP4398148B2 (ja)
AU (2) AU2386702A (ja)
CA (1) CA2436802A1 (ja)
DE (1) DE60110455T2 (ja)
GB (1) GB0029224D0 (ja)
TW (1) TW556001B (ja)
WO (1) WO2002044788A1 (ja)

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US10141950B2 (en) 2007-05-01 2018-11-27 The Texas A&M University System Low density parity check decoder
US10288480B2 (en) * 2015-06-02 2019-05-14 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Optical filter, optical device and method for determining a property of a substance by using an optical filter

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US6710922B2 (en) * 2001-12-07 2004-03-23 Nortel Networks Limited Optical filters
DE10328798A1 (de) * 2003-06-26 2005-01-20 Robert Bosch Gmbh Infrarot-Filter-Bauelement, insbesondere für einen Gasdetektor
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DE102017129069B3 (de) * 2017-12-06 2018-09-13 Laser Zentrum Hannover E.V. Optisches Element mit alternierenden Brechungsindexänderungen und dessen Verwendung
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EP4365651A1 (en) * 2022-11-01 2024-05-08 Leibniz-Institut für Astrophysik Potsdam (AIP) Method for the manufacture of an etalon and fiber-based etalon

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10141950B2 (en) 2007-05-01 2018-11-27 The Texas A&M University System Low density parity check decoder
US10288480B2 (en) * 2015-06-02 2019-05-14 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Optical filter, optical device and method for determining a property of a substance by using an optical filter

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WO2002044788A1 (en) 2002-06-06
JP4398148B2 (ja) 2010-01-13
DE60110455D1 (de) 2005-06-02
GB0029224D0 (en) 2001-01-17
TW556001B (en) 2003-10-01
EP1337888B1 (en) 2005-04-27
JP2009080509A (ja) 2009-04-16
JP2004514945A (ja) 2004-05-20
US20040057144A1 (en) 2004-03-25
AU2002223867B2 (en) 2005-02-03
EP1337888A1 (en) 2003-08-27
CA2436802A1 (en) 2002-06-06
AU2386702A (en) 2002-06-11
DE60110455T2 (de) 2006-02-23

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